A variety of materials are used to increase the density of well bore fluids. Well bore fluids are used for well bore stability, control the flow of gas, oil or water from the formation in order to prevent, for example, the flow or blow out of formation fluids or the collapse of the formation itself. Conventional weighting agents such as powdered barites and calcium carbonates (d50 10-30 μm) will settle in minutes over several inches when mixed with a carrier fluid such as water or oil. This can be largely predicted by Stokes Law. Therefore, in order to suspend the weighting agent particles, a gellant such as bentonite or a soluble polymer such as xanthan gum may be used to increase the viscosity of the carrier fluid. However, as more gellant is added to increase the suspension stability, the fluid viscosity (plastic viscosity) increases undesirably resulting in reduced pump ability among other fluid mechanical properties.
The sedimentation (or “sag”) of particulate weighting agents becomes more critical in well bores drilled at high angles from the vertical. For example, sag of one inch can result in a continuous column of reduced density fluid along the upper portion of the well bore wall. Such high angle wells are frequently drilled over large distances in order to access remote portions of an oil reservoir. In this case, it becomes even more critical to minimize a drilling fluid's plastic viscosity in order to reduce the pressure losses over the borehole length. This is also important in deep high pressure wells where high density well bore fluids are required. High viscosities can result in an increase in the effective density exerted by a circulating fluid against the formation, accounting for the pressure drop in the annulus above the point being considered, or the Equivalent Circulating Density. Such high viscosities can result in formation damage and fluid losses.
The conventional view held that reducing particle size in drilling fluids would lead to an undesirable increase in viscosity. However, it was determined that very finely ground particles (d50<2 μm and d90<4 μm) coated with a deflocculating agent or dispersant generated suspensions or slurries that reduced sag while the dispersant controlled the inter-particle interactions that produced lower Theological profiles. Thus, the combination of the fine particle size and the control of colloidal interactions reconciled the two objectives of lower viscosity and minimal sag.
To date, such finely ground, coated particles have been produced using stirred media mills. It is known, however, that such finely ground particles are a natural, although undesirable, byproduct of regular API weighting agent production. According to current API standards, particles having a effective diameter less than 6 microns, also known as “fines,” may make up no more than 30% by weight of the total weighting agent to be added to the drilling fluid. Thus, while it is acceptable to have fine particles in the weighting agent, it is preferred that the relative quantity of such particles be minimized. It would be beneficial to the production of the API weighting agent to harvest a substantial quantity of the fine particles created during production for use in a different product wherein such fine particles are desired.
A particle size distribution chart for the API weighting agent barite produced in a typical process is shown in FIG. 1. As can be seen within circle 600, nearly a third of the particles have a particle size of less than 6 μm. From a careful examination of the chart, one of skill in the art can see that particles less than 4 μm in size make up approximately 18% of the API Barite produced in this particular run. Thus, there exists a percentage of barite that is usable in alternative drilling fluids if it were separated from the API Barite. Such separation would simultaneously improve the quality of the API Barite, as the existence of such fine particles is undesirable in regular drilling fluids.
The concentration of fines in typical non-deviated drilling applications is problematic. An increase in fines results in an increase in chemical consumption for virgin drilling fluids, undesirable Theological properties while drilling, increased fluid loss, and increased reliance on mechanical separation equipment and ongoing dilution rates while drilling. This is universal for both water and oil or synthetic based drilling fluids. Therefore, the removal of fines is desirable. Such a low concentration of fines could reduce drilling costs, increase rate of penetration, reduce drag and incidents of stuck pipe.
Referring to FIG. 2, a typical process 500 for the production of commercial-grade weighting agent is depicted. Commercial-grade weighting agent is defined as weighting agent a maximum of 30% by weight of particles less than 6 microns and a maximum of 3% by weight of residue greater than 75 microns. The minimum effective diameter that weighting agent particles are permitted by API standards will be referred to hereinafter as the “minimum acceptable diameter.” The maximum effective diameter that weighting agent particles are permitted by API standards will be referred to hereinafter as the “maximum acceptable diameter.” It will be understood that the minimum and maximum acceptable diameters are determined by the API standards and do not reflect control limits as determined by statistical process control of an ongoing process.
In a precrushing step 510, relatively large pieces of weighting agent, typically barite or calcium carbonate, are released into a primary crusher to reduce the size of the larger pieces thereby obtaining a more uniform particle size going into the downstream processes. From the primary crusher, the particles are milled 520. Heat and additional crushing devices are used to reduce moisture and the particle size of the weighting agent even further. Typically, the secondary crusher utilizes heavy rollers that pulverize the weighting agent against an outer wall of the crusher.
The milled weighting agent is classified by size 530 in a classifier internal to the mill. Particles that have a size exceeding the acceptable maximum diameter remain in the mill for further milling 520. The particles that are less than the maximum size specified for commercial-grade weighting agent are discharged from the internal mill classifier 540 to a cyclone. The weighting agent entering the cyclone includes a relatively low concentration of undersized particles, known as “fines.”
In the cyclone, the larger, heavier particles fall to the bottom, where there is typically a rotary air lock valve, dual flapper gate valves, or other means for discharging predetermined quantities of weighting agent particles 550 separated in the cyclone. Some of the undersized fine particles, or fines, are carried to the bottom of the cyclone by larger particles to which the fines are attached or fall there individually. Predetermined quantities of weighting agent are released from the cyclone in bulk to a conveyer belt either periodically, such as by the rotary valve, or when the amount of weighting agent reaches sufficient weight, such as that needed to open the dual flapper gate valves.
A majority of the fines, are carried along with air currents from the cyclone into an adjacent baghouse rather than falling to the bottom of the cyclone. The baghouse retains a number of bags that filter the fine particles from air that is released from the process, thereby trapping the fine particles 560. At predetermined intervals, the bags are subjected to a sudden force that causes the fines collected thereon to be released to the bottom of the baghouse and onto the conveyer belt 570. The timing of the application of force necessary to release the fines is coordinated with the release of the larger particles from the cyclone to ensure that the overall quantity of weighting agent particles, coarse and fine, is within the tolerated limit of acceptable fines.
The discharge of larger weighting agent particles to the conveyor represents a majority of the overall flow rate onto the conveyor, while the discharge of fines to the conveyor represents a very small percentage of the flow rate on a mass basis. Although it is desirable to eliminate the fines from the commercial-grade weighting agent, disposal of the fines has heretofore been an issue environmentally and economically. The commercial-grade weighting agent released to the conveyor and the fines released to the conveyor are commingled and conveyed 580 to a packaging area where they are packaged 590 for distribution.
As commercial applications of fines have been discovered, processes to create fines from commercial-grade weighting agent or non-processed weighting agent have been evaluated. Given the fact that fine particles are created as part of the process to mill and package commercial-grade weighting agent, it would be an improvement to be able to modify existing processes and equipment to harvest a majority of the fines from the commercial-grade weighting agent process while producing a premium weighting agent product having a lower percentage of fines rather than reintroducing the fines to the commercial-grade weighting agent.